1. Field of the Invention
The present invention relates to a recording medium, and more particularly, to a recording medium having a high melting point recording layer, an information recording method thereof, and an apparatus and method for reproducing information therefrom.
2. Description of the Related Art
Conventional recording media can be classified into magneto-optical recording media or phase change recording media. In magneto-optical recording media, such as mini disks (MDs), information is read by detecting the rotation of a straight polarized light reflected from a magnetic film depending on the magnetic force and the magnetization direction of the magnetic film. The rotation of the reflected light is known as the “Kerr Effect”. In phase change recording media, such as digital versatile discs (DVDs), information is read based on the difference in reflectivity due to the different absorption coefficients of an optical constant between an amorphous recorded domain and a crystalline non-recorded domain of the recording medium.
Recently, many diversified methods of recording information using micro marks (pits), as in a phase change method, and reproducing information from the recording medium regardless of the diffraction limit have been suggested. The most interested one among these methods is a recording method using a super-resolution near-field structure, which is disclosed in Applied Physics Letters, Vol. 73, No. 15, October 1998, and Japanese Journal of Applied Physics, Vol. 39, Part I, No. 2B, 2000, pp. 980-981. A super-resolution near-field structure utilizes local surface plasmon generated in a special mask layer to reproduce information. The super-resolution near-field structure is classified as an antimony (Sb) transmission type which has an antimony mask layer that becomes transparent by laser irradiation when reproducing information from the recording medium or as a silver oxide decomposition type which has a silver oxide (AgOx) mask layer that decomposes into oxygen and silver, which acts as a scattering source inducing local plasmon.
FIG. 1 illustrates the structure of a recording medium using a conventional super-resolution near-field structure and the recording principle thereof. Such a structure as illustrated in FIG. 1 is referred to as “single-masked super-resolution near-field structure”.
As shown in FIG. 1, the recording medium includes a second dielectric layer 112-2 made of dielectric materials, for example, ZnS—SiO2, a recording layer 115 made of, for example, GeSbTe, a protective layer 114 made of dielectric materials, for example, ZnS—SiO2 or SiN, a mask layer 113 made of, for example, Sb or AgOx, a first dielectric layer 112-1 made of dielectric materials, for example, ZnS—SiO2 or SiN, and a transparent polycarbonate layer 111, which are sequentially stacked upon one another. When the mask layer 113 is made of Sb, SiN is used for the protective layer 114 and for the first dielectric layer 112-1. When the mask layer 113 is made of AgOx, ZnS—SiO2 is used for the protective layer 114 and for the first dielectric layer 112-1. The protective layer 114 prevents reaction between the recording layer 115 and the mask layer 113 and becomes a place where a near field acts when reproducing information. When reproducing the information, Sb of the mask layer 113 becomes transparent, and AgOx of the mask layer 113 decomposes into oxygen and silver, which acts as a scattering source inducing local plasmon.
The recording medium is irradiated with a laser beam of about 10-15 mW emitted from a laser source 117 through a focusing lens 118 to heat the recording layer 115 above 600° C. so that a laser-irradiated domain of the recording layer 115 becomes amorphous and has a smaller absorption coefficient k regardless of the change of refractive index n of an optical constant (n,k). In an irradiated domain of the Sb or AgOx mask layer 113, the crystalline structure of Sb changes or the quasi-reversible AgOx decomposes, generating a probe as a near-field structure pointing at a region of the recording layer 115. As a result, it is possible to reproduce information recorded on the recording medium as micro marks which are smaller in size than a diffraction limit of the laser used. Therefore, it is possible to reproduce information recorded in a high-density recording medium using such a super-resolution near-field structure regardless of a diffraction limit of the laser used.
FIG. 2 illustrates the structure of a recording medium using another super-resolution near-field structure and the recording principle thereof. Such a structure as illustrated in FIG. 2 with two mask layers is referred to as “double-masked super-resolution near-field structure” and provides improved performance over a single-masked super-resolution near-field structure.
As shown in FIG. 2, the recording medium includes a second dielectric layer 122-2 made of dielectric materials, for example, ZnS—SiO2, a second mask layer 123-2 made of, for example, Sb or AgOx, a second protective layer 124-2 made of dielectric materials, for example, ZnS—SiO2 or SiN, a recording layer 125 made of, for example, GeSbTe, a first protective layer 124-1 made of dielectric materials, ZnS—SiO2 or SiN, a first mask layer 123-1 made of, Sb or AgOx, a first dielectric layer 122-1 made of dielectric materials, for example, ZnS—SiO2 or SiN, and a transparent polycarbonate layer 121, which are sequentially stacked upon one another. When the first and second mask layers 123-1 and 123-2 are made of Sb, SiN is used for the first and second protective layers 124-1 and 124-2 and the first and second dielectric layers 122-1 and 122-2. When the first and second mask layers 123-1 and 123-2 are made of AgOx, ZnS—SiO2 is used for the first and second protective layers 124-1 and 124-2 and the first and second dielectric layers 122-1 and 122-2. The second mask layer 123-2 generates surface plasmon at a side of the recording medium opposite to the laser irradiation side. The first and second protective layers 124-1 and 124-2 prevent reaction between the recording layer 125 and the respective first and second mask layers 123-1 and 123-2. Particularly, the first protective layer 124-1 acts as a near field when reproducing information. When reproducing information, Sb of the first and second mask layers 123-1 and 123-2 becomes transparent, and AgOx of the first and second mask layers 123-1 and 123-2 decomposes into oxygen and silver, which acts as a scattering source inducing local plasmon.
The recording medium is irradiated with a laser beam of about 10-15 mW emitted from a laser source 117 through a focusing lens 118 to heat the recording layer 125 above 600° C. so that a laser-irradiated domain of the recording layer 125 becomes amorphous and has a smaller absorption coefficient k, regardless of the change of refractive index n of an optical constant (n,k). In an irradiated domain of the first and second mask layers 123-1 and 123-2, which are made of Sb or AgOx, the crystalline structure of Sb changes or the quasi-reversible AgOx decomposes, generating a probe as a near-field structure pointing at a region of the recording layer 125. As a result, it is possible to reproduce information recorded on the recording medium as micro marks which are smaller in size than a diffraction limit of the laser used. Therefore, it is possible to reproduce information recorded in a high-density recording medium using a super-resolution near-field structure regardless of a diffraction limit of the laser used.
However, in recording media having such a super-resolution near-field structure, since the mask layer and recording layer have similar transition temperatures, ensuring thermal stability to both the mask layer and the recording layer during information reproduction is important. Possible solutions to this problem include dropping the transition temperature of the mask layer and raising the transition temperature of the recording layer. However, it is not easy to induce a larger difference in transition temperature between the mask layer and the recording layer due to the nature of the materials forming the two layers.